For driving safety and comfort, it has long been necessary to control the reflectance of rearview mirrors in automobiles and other motor vehicles. At the driver's preference, the mirror reflectance can be altered from a highly reflective or “bright-mode” (BM) state to a less reflective “dark-mode” (DM) state. When other vehicles with bright headlights approach from behind at night, the rearview mirror is switched from its BM state to its DM state to minimize the headlight glare reflection from the rearview mirror since the annoying light intensity can hinder the driver's efforts to maneuver the vehicle safely. Conventionally, such adjustment was controlled manually by a mechanical lever that tilts the mirror reflecting angle slightly away from the driver. Later, the mechanical lever was controlled by electromechanical means, but still was done on demand by the driver. More recently, rearview mirrors are commercially available that automatically vary the reflectance of a mirror from BM to DM or vice versa using electrochromic (EC) tinting technology without having to move the mirror physically.
An EC mirror dims when a Direct Current (DC) is applied between the front and back electrodes of a mirror cell that contains a chemical electrolyte. The dimming mechanism is an electrochemical platting process that proceeds slowly. An EC mirror generally takes 6 to 10 seconds to change from its original BM to a less reflective deep green color in its DM. Due to the complex electrochemical platting processes with the EC technology, the response time depends critically upon mirror size. The larger the mirror's area, the longer it takes to alter the reflectance of the mirror. As a result, it is not practical to build a large EC mirror for applications such as commercial trucks. Furthermore, the electrical current requirement to change reflectance states of an EC mirror varies from 80 to 120 milliamperes. Due to this relatively high current consumption, EC mirrors in automotive applications must be wired directly to the main power source of the vehicle; i.e., the battery. Although deriving the power through the vehicle's battery may be acceptable for an Original Equipment Manufacturing (OEM) item, it is not viable for after-market mirrors where customers demand that the mirror be a self-sustained unit without any requirement of external wiring.
A need persists for an automatic darkening and glare reducing mirror that successfully addresses these and other shortcomings of the prior art. It is to the provision of such a mirror that the present invention is primarily directed.